Machine vision inspection systems and methods and aperture covers for use therewith

ABSTRACT

Machine vision inspection systems and methods for inspecting objects, such as objects with shiny surfaces, as well as illuminators with aperture covers for use therewith. Use of such an aperture cover eliminates the void (dark spot) in the illumination field that inherently results from the presence of an aperture through which an associated camera views an object to be inspected.

TECHNICAL FIELD

The present invention is directed to the field of machine visioninspection, particularly to the machine vision inspection of objects,such as objects with an irregular and/or specular surface.

BACKGROUND

Machine vision inspection of objects would be generally familiar to oneof skill in the art. Systems and methods of machine vision inspectiontypically use one or more cameras and, frequently, one or more sourcesof illumination. Illumination may be provided in various formsincluding, but not limited to, direct illumination, back lightillumination, ring illumination, horizontal illumination, coaxialillumination, and dome illumination.

When inspecting an object having a specular (shiny) surface, asuper-diffuse illumination environment is particularly effective. Of theaforementioned illumination types, it has been found that domeillumination works very well for inspecting such objects, as well asobjects having an irregular (e.g., convex, spherical, semi-spherical)shape. A machine vision inspection device that provides domeillumination is generally referred to as a dome illuminator.

As illustrated in FIG. 1, known dome illuminators typically include ahemispherical dome that overlies an object to be inspected (imaged). Anillumination source, such as a ring of LEDs, is located within the dome.The interior surface of the dome is typically painted or otherwisecoated with a diffusely reflective material such that light emitted bythe illumination source is reflected off the dome and onto the object tobe imaged.

Dome illuminators also include a central (axial) aperture that passesthrough the dome to provide a viewing window for an associated camera.The camera is focused on the upwardly facing surface of the object to beimaged, which surface is illuminated by the light being reflected off ofthe dome interior. An unfortunate side-effect of this design, however,is that the central aperture appears in reflection as a dark spot on thesurface being imaged. This may result, for example, in an inability toread a code imprinted on the surface being imaged or an inability to seeother features thereof.

A known solution to this problem is a device commonly referred to as aCloudy Day® illuminator (CDI). A CDI typically includes a domeilluminator with a second co-axial light source that further illuminatesan object to be inspected by projecting light through the central domeaperture. The brightness of the dome and co-axial light sources arebalanced until they appear of equal brightness in reflection from theobject surface being inspected.

While a CDI is an improvement over a typical dome illuminator, CDIs areexpensive. Further, the addition of the co-axial light source adds tothe size, and mechanical and electrical complexity of the domeilluminator, and also renders typical CDIs quite bulky. CDIs also employa flat or curved beam splitter arrangement, which is usually fragile anddifficult to clean.

It can be understood from the foregoing commentary that it would bedesirable to provide a simple, cost-effective and compact machine visioninspection device that is capable of accurately imaging (inspecting) theentire specular surface of an object. Embodiments of the presentinvention are such devices.

SUMMARY

Embodiments of the invention provide substantially the same illuminationuniformity as a CDI, but eliminate the complexity, added cost, and size.Embodiments of the invention are also able to eliminate the reflecteddark spot problem referred to above with respect to the use of a typicaldome illuminator. This is accomplished by making the reflected image ofthe central aperture of an illumination housing effectively disappear bycovering the aperture with a diffusely reflective aperture cover.

An aperture cover according to the invention is, generally speaking, acover adapted for placement over the central aperture of an illuminationhousing, with the cover containing an array of diffusely reflectingfeatures interspersed between an array of very narrow slits that allowlight to pass therethrough. The pattern of diffusely reflecting featuresand gaps may be arranged in a regular or irregular pattern.

With the cover in place over the central aperture, an associated camerais located close to the aperture cover. If the numerical aperture of thecamera lens is held substantially wide open, the features of theaperture cover can be made to disappear in reflection, and the typicaldark spot problem is eliminated. Embodiments of the invention can befurther optimized by matching as closely as possible the reflectivity ofthe illumination housing interior and the aperture cover. The cameraside of an aperture cover may also be coated with a non-reflective orsubstantially non-reflective material.

Other aspects and features of the invention will become apparent tothose skilled in the art upon review of the following detaileddescription of exemplary embodiments along with the accompanying drawingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

In addition to the features mentioned above, other aspects of thepresent invention will be readily apparent from the followingdescriptions of the drawings and exemplary embodiments, wherein likereference numerals across the several views refer to identical orequivalent features, and wherein:

FIG. 1 is a partially cutaway view of a typical dome illuminator;

FIG. 2 schematically illustrates an exemplary embodiment of a typicalcloudy day illuminator;

FIGS. 3A-3C show exemplary embodiments of aperture covers having regularand irregular patterns of the invention, for placement over the centralaperture of an illumination housing;

FIG. 4 is a simplified cross-sectional elevation view of a machinevision inspection device of the invention with an exemplary aperturecover located over the central aperture thereof; and

FIG. 5 is a bottom view of the machine vision inspection system of FIG.4, wherein slits in the sieve aperture cover can be seen through thecentral aperture in the illumination housing.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

As discussed above, known dome illumination systems are used to inspectspecular surfaces of objects of interest, but all suffer from aninherent void in the illumination field that results from the aperturethrough which the camera views the object being inspected. This voidappears as a dark feature. “Healing” this void in the illuminationfield, to make it truly continuous, may be accomplished using a knownCloudy Day® illuminator (CDI) that makes use of a slanted or curved beamsplitter illuminated by a secondary light source, which is balanced inbrightness to match the surrounding illumination field. However, CDIsalso have certain drawbacks, as mentioned above. Examples of a typicaldome illuminator and a CDI are respectively depicted in FIG. 1 and FIG.2.

A typical dome illuminator 5 may be observed in FIG. 1. As shown, thedome illuminator 5 includes a substantially hollow illumination housing10 with an open bottom. The interior of the illumination housing 10forms a hemispherical diffuser 15. An illumination source 20, such as aring light, a plurality of linear light sources, or an array of discreteLED light sources, is located near the base of the illumination housing10 and is adapted to provide the illumination necessary to carry out aninspection process on an object 25 located below the illuminationhousing 10 and diffuser 15 portion thereof.

As can also be observed in FIG. 1, light rays 30 from the illuminationsource 20 are directed upward, where they are reflected withsubstantially equal brightness by the diffuser 15 back toward the objectto be inspected. The desired surface of the object of interest 25 isthus indirectly illuminated 35 by the illumination source 20. A centralaperture 40 passes through the illumination housing/diffuser 10, 15 suchthat the illuminated surface of the object of interest is made visibleto an associated camera 45 located above the central aperture.

As described previously, a problem with such a dome illuminator is thatthe central aperture 40 will appear as a dark feature in the reflectionof the object surfaces being imaged. Consequently, codes and/or otherfeatures of interest on the object surfaces being imaged may not be seenby the camera due to inadequate contrast.

A typical cloudy day illuminator (CDI) 50 designed to overcome theaforementioned problem with a dome illuminator is schematicallyillustrated in FIG. 2. As with the dome illuminator 5 exemplified inFIG. 1, the CDI 50 also includes a substantially hollow illuminationhousing 55 with an open bottom, the interior of the illumination housingforming a hemispherical diffuser 60. An illumination source 65, whichmay again be a ring light, is located near the base of the illuminationhousing 55 and is adapted to provide a portion of the illuminationnecessary to carry out an inspection process on an object of interest 70located below the illumination housing and diffuser 60 portion thereof.

In comparison to the dome illuminator, however, the CDI 50 includes asecond illumination source 75, which may be generally referred to as anon-axis or co-axial illumination source. The second illumination source75 projects light through a beam splitter 80, which directs the lightthrough a central aperture 85 in the illumination housing/diffuser 55,60 and onto the surface of the object of interest 70.

As with the dome illuminator of FIG. 1, light rays 90 from theillumination source 65 are directed upward, where they are reflectedwith substantially equal brightness by the diffuser 60 back toward theobject of interest 70. The desired surface of the object of interest 70is thus indirectly illuminated by the illumination source 65 anddirectly illuminated by the second illumination source 75. Light fromthe object surface is reflected upward through the central aperture 85to an associated camera 95 located above the illumination housing 55where it forms part of the image. A clear (e.g., plastic) aperture 100may be provided in the beam splitter arrangement to permit viewing ofthe object image by the camera 85.

As should be apparent from an observation of FIG. 2 and the abovedescription, the additional components required to construct a CDI alsomake a CDI much more expensive than a typical dome illuminator.Furthermore, because of the beam splitter and associated structurecommonly installed on the dome of a CDI, it is not possible to locatethe inspection camera as close to the illumination housing as might bepossible without the presence of such a secondary illumination source.This has the result of making a typical CDI quite bulky in comparison toa typical dome illuminator. Still further, the beam splitterarrangements commonly employed with a CDI are fragile and/or difficultto clean.

Therefore, embodiments of the invention are directed to machine visioninspection systems and methods that are inexpensive and compact incomparison to a CDI, but that produces CDI-like inspection results byeliminating the inherent void in the illumination field associated witha typical dome illumination-type devices. Generally speaking, this isaccomplished through the use of an inspection device having anillumination housing equipped with a novel aperture cover that functionsto fill the aperture void (dark spot) inherent to typical domeilluminators with a discontinuous pattern of opaque but diffuselyreflecting features of very narrow dimension. These features may bethought of as sieve-like in nature with respect to the aperture coversand may be, for example, a series of slits which, when viewed inreflection from a specular surface of an object of interest, appearout-of-focus and blend into the surrounding illumination field.

Several exemplary and non-limiting aperture cover embodiments havingvarious patterns of diffusely reflective features are shown in FIGS.3A-3C. It is to be understood that the roughness (smoothness) of thesurface being imaged, typically expressed as a root mean squared (RMS)dimension, largely determines the dimensions of the diffusely reflectivefeatures of the aperture cover. That is, the roughness of the surfacebeing imaged will typically dictate that the diffusely reflectivefeatures fall within some limited range of dimensions in order for thediffusely reflective features to appear “invisible” to the camera inreflection off the surface. Generally speaking, the rougher the surfacebeing imaged, the larger the diffusely reflective features may be.However, there is also a practical benefit to using smaller sieveaperture features—namely that surfaces having a greater range of surfaceroughness can be reliably imaged. Consequently, the diffusely reflectivefeatures of the exemplary aperture covers depicted in FIGS. 3A-3C arestated to fall within a particular dimension range or to be of aparticular dimension. These dimension ranges and/or dimensions have beenfound to be particularly effective when used to image common surfaces tobe inspected, such as the lids of typical metal cans. Nonetheless, otherdiffusely reflective feature dimension ranges or dimensions may be moresuitable to the imaging of other surfaces.

In FIG. 3A, a first exemplary aperture cover 105 is shown to include asheet of material 110 having a pattern of diffusely reflective featurescomprising a series of parallel, closely spaced, narrow slits 115 thatpass therethrough. The number of slits 115, the width of the slits andthe spacing between the slits may vary as explained above. In onenon-limiting example, the width of the slits 115 may be betweenapproximately 0.01 and 2.0 millimeters, more particularly between about0.1 and 0.5 millimeters, and even more particularly, about 0.25millimeters.

A second exemplary aperture cover 120 is shown in FIG. 3B to include asheet of material 125 having a pattern of diffusely reflective featurescomprising a series of regularly recurring and substantiallyequidistantly-spaced holes 130 that pass therethrough. In thisparticular example, the holes 130 are shown to be substantiallyrectangular in shape, but various other shapes and other patterns arealso possible. For example, such a regularly occurring pattern may takethe form of a tessellation. The number of holes 130, the dimension(s) ofthe holes and the spacing between the holes may vary as explained above.In one non-limiting example, the minimum dimension of the holes 130 maybe between approximately 0.01 and 2.0 millimeters, more particularlybetween about 0.1 and 0.5 millimeters, and even more particularly, about0.25 millimeters.

A third exemplary aperture cover 135 is shown in FIG. 3C to include asheet of material 140 having a pattern of diffusely reflective featurescomprising a multitude of non-repeating, substantially non-parallel andirregularly spaced holes 145 of various shape that pass therethrough. Inthis particular example, the holes 145 are shown to include variousshapes (e.g., ellipses and squares), but various other shapes andcombinations of shapes are also possible. The number of holes 145, thedimension(s) of the holes and the spacing between the holes may againvary as explained above. In one non-limiting example, the minimumdimension of the holes 145 may be between approximately 0.01 and 2.0millimeters, more particularly between about 0.1 and 0.5 millimeters,and even more particularly, about 0.25 millimeters.

The size and shape of the exemplary aperture covers 105, 120, 135 ofFIGS. 3A-3B are provided for purposes of illustration only and are notto be construed as limiting the invention to the relative dimensionsand/or shapes shown. Aperture covers of other shapes such as, forexample, circular shapes, could also be employed. Likewise, the patternof the diffusely reflective features located in a given aperture cover,as well as the dimensions of the features, may vary as long as theeffect thereof enables the aperture cover material to be renderedout-of-focus and, hence, substantially invisible to the inspectioncamera as described above and as illustrated and described in moredetail below with respect to the embodiments of FIGS. 4-5. Similarly,aperture cover embodiments of the invention are not limited tomanufacture from a particular material. The aperture cover pattern mayalso be formed directly in the dome material rather than as a separatepiece attached thereto.

A simplified cross-sectional elevation view of an exemplary embodimentof an inspection device 150 according to the invention is illustrated inFIG. 4. As shown, the inspection device 150 is similar to a typical domeilluminator and includes a substantially hollow body (illuminationhousing) portion 155 with an open bottom. The interior of theillumination housing 155 forms, in this case, a dome-shaped (e.g.,hemispherical) diffusely reflective surface (diffuser) 160. Otherdiffuser shapes may also be possible in other embodiments as long as theselected shape results in a proper reflection of light onto an object ofinterest. For example, the reflective surface may be flat in certainembodiments.

An illumination source 165, such as a ring light, a plurality of linearlight sources, or an array of discrete (e.g., point) light sources suchas LEDs, is located near the base of the housing 160 and is adapted toprovide the illumination necessary to carry out an inspection process onan object of interest 170 located below the illumination housing 155 anddiffuser 160.

As previously described with respect to the known dome illuminator 5 ofFIG. 1, light rays 175 from the illumination source 165 are directedupward, where they are reflected with substantially equal brightness bythe diffuser 160 back toward the object of interest 170. The desiredsurface of the object of interest 170 is thus indirectly illuminated bythe illumination source 165.

As also previously described with respect to the known dome illuminator5 of FIG. 1, a central aperture 180 passes axially through the diffuser160 and illumination housing 155 to provide an opening through which theobject of interest 170 may be viewed with a camera 185. In a traditionaldome illuminator, this would result in the central aperture appearing inreflection as a dark spot on the specular surface being inspected.

In order to overcome this dark spot problem, it can be observed in FIG.4 that an exemplary aperture cover 190 of the invention is located overthe central aperture 180 of the inspection device 150. The aperturecover 190 may be one of the exemplary aperture covers 105, 120, 135depicted in FIGS. 3A-3C, or it may be of a different design. In anycase, the camera 185 lens is located close to the aperture cover 190 andis operated at a wide aperture setting. As a result, the features of theaperture cover 190 are rendered out-of-focus and, hence, invisible tothe inspection camera 185, despite the aperture cover material beingsubstantially opaque. At the same time, the diffusely reflectingfeatures of the aperture cover 190, when viewed in reflection from thespecular surface of the object being inspected 170, appear out-of-focusand cause the reflected aperture 180 to blend into the surroundingillumination field, thereby eliminating the aforementioned dark spotproblem. This allows for a more complete and accurate inspection of thespecular surface of the object of interest.

For purposes of further illustration, an enlarged bottom view of theinspection device 150 of FIG. 4 is illustrated in FIG. 5. In this view,a portion of the aperture cover 190 is visible through the centralaperture 180 in the illumination housing 155 of the inspection device150. It can be observed that this particular exemplary aperture cover190 is comprised of a sheet of material 195 that includes a series ofparallel, closely spaced, narrow slits 200 that pass therethrough—muchlike the exemplary aperture cover 110 depicted in FIG. 3A. Such an arrayof narrow slits may be replaced by any pattern, regular or irregular,wherein the size of reflecting features and the gaps therebetween aresmall enough to be rendered substantially invisible in reflection asseen by the camera.

As previously explained, with the aperture cover 190 in place over thecentral aperture 180, the associated camera 185 is located close to theaperture cover. If the lens aperture of the camera 185 is heldsufficiently open, the pattern of slits 200 in the aperture cover 190may be made to disappear in reflection by being out of focus, and thedark spot that would typically represent the reflected image of thecentral aperture 180 is made to have substantially the same brightnessas the surrounding diffuser 160 surface or to appear sufficientlysimilar in brightness to allow for reliable detection and imaging ofsurface features to be performed.

The results that can be produced by such an inspection device are quiteadequate for typical optical character recognition (OCR) algorithms, andmay be as good as or nearly as good as the results provided by a morecomplex and costly CDI. Additionally, inspection device embodiments ofthe invention can be further optimized by closely matching thereflectivity of the diffuser and the aperture cover. For example, thereflectivity of the diffuser may be slightly reduced and thereflectivity of the aperture cover material may be slightly increased,such that the apparent brightness in out-of-focus reflection of thediffuser and the aperture cover can be made to match as precisely aspossible. Although not essential, the camera side of an aperture coverof the invention may also be coated with a non-reflective orsubstantially non-reflective material.

It should be understood that the choice of relative dome and aperturecover reflectivity may be flexible based on various factors, such asthose described below, but obviously reaches an optimum when theout-of-focus apparent brightness of the aperture cover matches theapparent brightness of the surrounding surface. It has also beendiscovered that a high quality vision system using good edge-detectionalgorithms is relatively insensitive to a brightness mismatch incomparison to a typically cheaper but faster vision system usingsimplified brightness-threshold-based character segmentation algorithms.A better vision system, like a CIV, will thus be more insensitive to thepotential difference in reflectivity between the diffuser and theaperture cover. Consequently, when used to examine common surfaces suchas the lids of metal cans, it has been found, depending on the visionsystem employed, the range of surface roughness of the lids, etc., thatthe reflectivity of the diffuser may be within about ±75% of the averagereflectivity of the aperture cover. In some embodiments, it may bepreferable that the reflectivity of the diffuser is within about ±30% ofthe average reflectivity of the aperture cover and, in yet otherembodiments, it may be preferable that the reflectivity of the diffuseris within about ±10% of the average reflectivity of the aperture cover.

While certain exemplary embodiments of the present invention aredescribed in detail above, the scope of the invention is not to beconsidered limited by such disclosure, and modifications are possiblewithout departing from the spirit of the invention as evidenced by thefollowing claims:

What is claimed is:
 1. A dome illuminator for the illumination of anobject of interest, comprising: a substantially hollow housing with anopen bottom, the interior of the housing forming a dome-shaped diffusercapable of reflecting light toward the open bottom thereof; anillumination source located near the open bottom of the housing andoriented to direct light onto the diffuser to create an illuminationfield; a central aperture providing a view, through the open bottom ofthe housing, of the object of interest; and an aperture cover located onthe housing exterior and over the central aperture, the aperture coverhaving a discontinuous pattern of substantially opaque but diffuselyreflective features oriented toward the open bottom of the housing. 2.The dome illuminator of claim 1, wherein the discontinuous pattern ofsubstantially opaque but diffusely reflective features of the aperturecover comprises a plurality of substantially parallel and spaced apartnarrow slits.
 3. The dome illuminator of claim 2, wherein the width ofeach slit is between approximately 0.01-2.0 millimeters in minimumdimension.
 4. The dome illuminator of claim 2, wherein the width of eachslit is between approximately 0.1-0.5 millimeters in minimum dimension.5. The dome illuminator of claim 2, wherein the width of each slit isapproximately 0.25 millimeters in minimum dimension.
 6. The domeilluminator of claim 1, wherein the discontinuous pattern ofsubstantially opaque but reflective features of the aperture covercomprises an irregular non-parallel or non-repeating pattern of suchfeatures.
 7. The dome illuminator of claim 1, wherein the discontinuouspattern of substantially opaque but reflective features of the aperturecover comprises a regular repeating tessellation or pattern.
 8. The domeilluminator of claim 1, wherein the reflectivity of the diffuser iswithin about ±75% of the average reflectivity of the aperture cover. 9.The dome illuminator of claim 1, wherein the reflectivity of thediffuser is within about ±30% of the average reflectivity of theaperture cover.
 10. The dome illuminator of claim 1, wherein thereflectivity of the diffuser is within about ±10% of the averagereflectivity of the aperture cover.
 11. The dome illuminator of claim 1,wherein a camera side of the aperture cover is coated with anon-reflective, substantially non-reflective, or substantially opaquematerial.